The present invention relates to a thermal conductivity detector which is used for a gas chromatograph for performing gas analysis by utilizing the difference in adsorptivity between a filler and a gas filled in, e.g., a column, and is designed to detect the thermal conductivity of the gas so as to perform the gas analysis.
In a petrochemical process, a steel process, or the like, a gas chromatograph has been generally used as an analyzer for analyzing the components of a process gas, monitoring each step in a process on the basis of the analysis result, and performing various types of control operations.
In such a gas chromatograph, a column is filled with a powder, as a stationary phase, such as active carbon, active alumina, or molecular sieves having a uniform grain size. The type of powder differs depending on the sample gas. The components of a sample gas are separated from each other by utilizing the difference in moving rate based on the difference in adsorptivity or the difference in distribution coefficient between the stationary phase and each component of the sample gas. Each component is then detected by a detector such as a thermal conductivity detector (TCD).
As such a thermal conductivity detector, a temperature-sensitive element such as a thermistor has been used. A small integrated detector has recently been proposed by Jhon H. Jerman et al. (U.S. Pat. No. 4,471,647).
This detector is constituted by an elongated metal film resistor, a membrane holding the metal film resistor and having a plurality of holes to allow a gas to flow on both the sides of the membrane, and a means for holding the membrane and the metal film resistor
These thermal conductivity detectors are driven by, e.g., a constant current to detect the ratios of different types of gases by detecting differences in heat radiation ratio from the detectors into the gases, caused by differences in thermal conductivity between the gases, in terms of voltages.
In the detector disclosed in U.S. Pat. No. 4,471,647, since a plurality of holes are formed in the membrane at positions near the detector terminals to allow a gas to flow on both the sides of the membrane, when the gas flows on both the sides of the membrane, the detector may be adversely affected by the gas flows to cause an error or may detect disturbance of the flow, resulting in measurement error. In addition, since a large amount of heat generated by the detector is dissipated between the membrane and the means for holding the membrane, accurate measurement cannot be performed. Furthermore, when particles such as dust and mist particles are trapped in the plurality of holes, the detection sensitivity of the sensor deteriorates.